1. Field of the Invention
The present invention relates to improvements in methods and apparatus for forming seals between film materials, such as in packaging.
2. Description of the Background Art
Bags for potato chips or other snack food products typically are formed, filled and sealed in a substantially continuous operation. To produce a bag, polymeric sheet material typically is formed into a tube by bringing the edges of the material into engagement as the material passes over a tube-forming collar. A portion of the tube corresponding to an end of a bag is gripped and sealed by jaws to form a closed-end tube. The jaws move downward, pulling the closed-end tube past a device which seals the engaged edges of the tube between a sealing die and a back-up plate by application of heat and pressure to the edges to form a back seal along the closed-end tube. The closed-end tube may be filled with product from a suitable device at substantially the same time that it is being back-sealed, or immediately thereafter. The jaws then release the sealed end and move upward along the length of the closed-end tube. The jaws stop and close together at a point corresponding to the opposite end of a product-filled bag to seal the end of the tube containing product to form the filled bag. The jaws simultaneously seal the end of the next tube and then move downward to effect the back seal of the newly-formed, close-end tube. The formed bag may be separated from the continuous material when the jaws are at their lowermost position.
During movement of the jaws downwardly while gripping the closed-end tube, the engaged edges of the tube are moved quickly past the back-sealer to form a "slip" area of the back seal. However, when the tube remains stationary after the jaws release the sealed end and move upward along the length of the closed-end tube, a portion of the engaged edges of the tube remain stationary, and within the sealing device for considerably longer than during movement of the tube, to form a "dwell" area of the back seal. Because of this variation in the residence time of tube edges within the back sealing device, the quality of the "dwell" and the "slip" areas of the seal often are inconsistent, particularly with certain types of film material. This may result in defectively sealed bags.
Defectively sealed bags often go undetected until the product moves into retail commerce. The customer may buy the package and be quite unaware that it has a defective seal. The faulty seal, however, may have caused the product to lose freshness and the customer is dissatisfied. Also, the customer may return the product to the store for exchange or a refund. Since by this time the whole chain of commercial transactions has occurred, the refund or exchange is generally accounted for by some type of back transactions giving credit to the retailer, route man and the like until reaching the product manufacturing stage. This is a costly and time-consuming process. Accordingly, there remains a need in the art for methods and apparatus for forming consistently high quality back seals in form and fill machines.